Much as I suspected, although you'd think such an unfortunate limitation
would be documented somewhere.

A probably simple way would be:
- set default to -fno-exceptions,
- print a warning if -fexceptions is specified.

Then compilation fails if exceptions or C++ standard library are used.
Option -fexception could be enabled if exceptions are used only to
handle fatal errors like allocation or assertion failures which is the
case for the standard library.

I honestly don't have the time or expertise to implement this right now,
so I'm afraid this is an upstream case of PTC.